This invention pertains generally to pulse compression radar systems and particularly to improvements in the manner in which echo signals may be digitally processed in the receiver of such a system.
It is well known in the art that any selected uncompressed echo signal received by a pulse compression radar may be processed in a matched filter to derive a compressed echo signal. Whether the matched filter operates in the time domain or the frequency domain, the compressed echo signal ultimately produced following any known matched filter technique is accomplished by range, or time, sidelobes. For proper operation, regardless of the amplitude of any selected uncompressed echo signal, it is then necessary to process the compressed echo signal and the accompanying range sidelobes to reduce the amplitude of the undesirable sidelobes without unduly degrading the desired compressed echo signal.
It is known that the amplitude of range sidelobes may be selectively attenuated by passing the compressed echo signal and the range sidelobes through a weighting circuit. Thus, it is known to use a conventional weighting circuit, as a Taylor or a Hamming weighting circuit, to reduce the amount of energy in the sidelobes without unduly widening the compressed echo signal. Such a technique is adequate so long as, in the frequency domain, the envelope of the signal out of the matched filter is a rectangle of constant amplitude corresponding to the familiar sin X/X distribution in the time domain.
Unfortunately, if the radar transmits a frequency modulated, or "chirp," pulse, conventional weighting techniques are not fully effective in reducing the amplitude of sidelobes. The reason for the lack of effectiveness is that the frequency spectrum of the signal out of the matched filter only approaches a sin X/X distribution. It follows, then, that, if a weighting technique for a sin X/X distribution is applied, the sidelobes cannot be reduced as much as possible.
The difference between the frequency spectrum of the signal out of the matched filter in a pulse compression radar using a chirp pulse and a sin X/X spectrum is due to the fact that the frequency spectrum of a chirp pulse contains amplitude ripples. These perturbations, which are determined by the length of the chirp pulse and the change in frequency of the modulation signal, are sometimes referred to as "Fresnel" ripples. That is, their amplitude and frequency may be described by formulas adapted from the Fresnel diffraction formulas for light waves. The presence of Fresnel ripples is particularly detrimental when pulse compression is accomplished by digital processing including Fourier transform techniques. That is, if an uncompressed echo signal is sampled, the samples converted to digital form and passed through a Discrete Fourier transform circuit (D.F.T.) to derive the frequency spectrum of the uncompressed echo signal and then such spectrum is multiplied, point by point, by the complex conjugate of the D.E.T. of the transmitted chirp pulse, it has been found that the range sidelobes of the resulting compressed signal remain prominent and may not be adequately reduced by any conventional weighting circuit.